Added force option to tools.cmake.cmaketoolchain:extra_variables

Changelog: Feature: Added force option to tools.cmake.cmaketoolchain:extra_variables. Docs: https://github.com/conan-io/docs/pull/3774

Added support for setting FORCE attribute to a cache variable from tools.cmake.cmaketoolchain:extra_variables Feature originally requested here

Improved error messages to make it more intuitive

[x] Refer to the issue that supports this Pull Request.
[x] If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
[x] I've read the Contributing guide.
[x] I've followed the PEP8 style guides for Python code.
[x] I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

conan-io / conan